An attached matter detector, which irradiates a plate-like transparent member such as a windshield of a vehicle by a light source, images the reflection light from the plate-like transparent member through an optical filter transmitting the wavelength of the light irradiated from the light source by an imaging element, and detects attached matter such as a raindrop attached on the surface of the plate-like transparent member, is described in Patent Document 1 (Japanese Patent Application Publication No. 2005-195566).
By providing such an optical filter, the attached matter on the plate-like transparent member can be detected without being affected by disturbance light such as sunlight or peripheral light.
However, in the attached matter detector described in Patent Document 1, disturbance light having a wavelength which is the same as that of the light emitted from the light source transmits through the optical filter, so that the disturbance light is false-detected as attached matter. An attached matter detector, which images a light-out image when a light source is turned off and a light-up image when a light source is turned on, calculates a brightness difference between the light-out and light-up images, and detects attached matter such as a raindrop if the brightness difference exceeds a threshold, is described in Patent Document 2 (Japanese Patent Application Publication No. 2005-195569). As described above, by calculating the brightness difference between the light-out and light-up images, disturbance light is eliminated, so that the reflection light component from the light source can be extracted. In this way, the attached matter can be effectively detected.
However, in the above attached matter detector, so-called flare light reflected from a housing of a device may enter into the imaging element when the imaging is performed with the turned-on light source. For this reason, the flare light remains in the difference information between the light-out and light-up images as noise in addition to the light reflected by attached matter. As a result, the attached matter having a brightness value which is approximately the same as that of the flare light is buried in the noise, and can not be detected. Therefore, sufficient detection accuracy can not be obtained.